Spokes are an often neglected, yet important part of your wheel. Most spokes break from the fatigue caused by the change in tension as the wheels turn round. More often, broken spokes will occur in the back wheel because back wheels carry more weight. And, if one spoke breaks, it's generally a signal that others may not be far behind.
*It's always a good idea to check your spokes regularly, and to keep a few extra (of the right length) on hand.